For the vinaigrette; combine blueberries, sugar and 2 tablespoons of vinegar in a small saucepan. Stir over medium-low heat for 2 minutes or until sugar has dissolved. Use a fork to crush the berries. Bring to simmer for 3 minutes. Remove from the heat. Strain while warm and using a spoon, press as much mixture through sieve at possible. Cool for 15 minutes. Add oil, horseradish cream and remaining vinegar. Season then whisk until well combined. Refrigerate until ready to serve.
Preheat oven to 200°C fan forced. Cut the baguette into 18 slices and spray both sides with oil. Place in a single layer on a baking tray. Bake, 4-5 minutes, or until lightly toasted. Remove from the oven, top each baguette slice with a slice of brie. Bake for a further 3-5 minutes, or until cheese is slightly melted.
Scatter the spinach, rocket, prawns, cashews and blackberries on a serving platter or board. Drizzle over a little vinaigrette and toss gently to coat. Top with brie croutons. Serve with remaining vinaigrette.